May 7, 2003
09:45 AM (EDT)

News Release Number: STScI-2003-15

Deepest View of Space Yields Young Stars in Andromeda Halo

May 7, 2003: Relying on the deepest visible-light images ever taken in space,
astronomers using NASA's Hubble Space Telescope (HST) have reliably measured
the age of the spherical halo of stars surrounding the neighboring
Andromeda galaxy (M31). To their surprise, they have discovered that
approximately one-third of the stars in Andromeda's halo formed only 6
to 8 billion years ago. That's a far cry from the 11-to-13 billion-year age of the stars in the Milky Way's halo.

Q & A: Understanding the Discovery

1.
Why is there a difference in the age range of the stars in the Andromeda and Milky Way halos?

Astronomers think that the collision with another large galaxy or the
ravaging of several smaller galaxies scattered the young stars into
Andromeda's halo. The newly discovered younger stars in Andromeda's halo
are richer in heavier elements than those in our Milky Way's halo, or in
most of the small dwarf galaxies that surround the Milky Way. The stars' age spread and chemical make-up suggests three possibilities: (1) Collisions destroyed
the young disk of Andromeda and dispersed many of its stars into the
halo; (2) a single collision destroyed a relatively massive invading
galaxy and dispersed its stars and some of Andromeda's disk stars into
the halo; and/or (3) many stars formed during the collision itself.
Astronomers say it will take more detailed observations to unravel the
evidence of these early cataclysmic events.

2.
Why do astronomers call these observations the "deepest visible-light images ever taken in space"?

Astronomers needed to make several hundred exposures to survey the
bright and the faint stars in Andromeda's halo. The image shown here was made from 250 separate exposures. The observations were taken from Dec. 2, 2002 to Jan. 11, 2003 with the
Advanced Camera for Surveys (ACS), but the total exposure time
was 3.5 days. Astronomers needed the lengthy observations
because the halo's ordinary Sun-like stars are very faint. In fact,
before using the ACS to tackle the halo stars, astronomers had observed
only the brightest ones. The sharp eyes of the ACS, however, uncovered
about 300,000 stars that astronomers had never seen before.